The invention relates to a screw extruder for working plastic compositions, especially polymers and rubber mixtures, consisting of a cylinder in which a motor-driven screw is disposed, the cylinder of which is provided with pins which are arrayed in several planes perpendicular to the cylinder axis and can be inserted radially through openings in the cylinder wall, and the screw is provided in the spirals of the screw with gaps disposed in the planes of the pins.
Such screw extruders are disclosed in German Patent 137,813 and U.S. Pat. No. 1,848,236. The pins extending radially into the extruder cylinder improve the kneading and mixing of the material, and in many cases they have made such kneading and mixing possible for the first time.
When the screw is changed and when the extruder is to be cleaned these pins have to be removed and then reinstalled. When the pins are threaded in place it becomes possible to adjust their depth, which is done for the purpose of reducing the screw resistance by having the pins reach less deeply into the cylinder and to improve the kneading and mixing action by setting the pins deeper.
Occasionally there is a desire to be able to vary the pin depth rapidly and without consuming too much time between zero and maximum for reasons relating to the process. Pin extruders for working rubber mixtures which are not preheated have attained great economical importance when constructed in accordance with German Patent 22 35 784. The mechanical fine pin adjustment operating with spiral plates is rarely used, however, for reasons of cost. The movement of, for example, eight pins in a single pin plane also requires considerable power and thus causes great wear on the adjusting means. For the numerous pin planes, amounting to about six to sixteen planes, this adjustment is furthermore quite time-consuming. Therefore there is a need for a rapid adjustment of the pins for the purpose of losing as little time as possible when the screw has to be replaced for technical reasons with one of a different shape, or because it is worn, or for cleaning purposes.
German Patent 35 03 911 has disclosed a hydraulic pin adjustment which permits the pins to be driven all the way in and backed all the way out, each pin being constituted by the plunger of a hydraulic cylinder. For each pin a separate hydraulic cylinder is provided. The possibility of achieving specific, defined pin depth settings has been practically unattainable on account of the immensely great complexity of the controls required. Even the complete plunging and extraction of the pins is so structurally complex that such hydraulic pin control has not become established to any great extent.
A pin adjuster that appears to be mechanically very simple, and which theoretically also offers the possibility of any desired depth adjustment, has been disclosed by German Patent 35 06 424. The disadvantage of this type of construction using Kellen drives for the screw movement producing the axial movement of the pins lies in the fact that extremely great friction is involved, which renders unfunctional any design based on this principle, and therefore it has never been used in actual practice.
The present invention avoids the disadvantages of the state of the art. It is the aim of the invention to create a practically usable and economical possibility for rapid pin movement and depth adjustment on screw extruders.